How can I make sure all instances of an object get destroyed, despite heiarchy?

One simple way that you could do this would be by enabling FindFirstChild()'s second parameter, like so:

game.Workspace:FindFirstChild("w", true)

The second parameter is a boolean (true / false) value that, when true, makes the function search through every descendent of the defined instance, which in this case is Workspace.

By default, the second parameter is set to false, so it won't work without enabling it like this.

If no valid instance is found with the defined name, then the function returns nil.

Using this, we can create a while loop (or a repeat loop), and having it repeat until all instances have been deleted. That would look something like this:

while game.Workspace:FindFirstChild("w", true) do -- Searches every instance in Workspace
    game.Workspace:FindFirstChild("w", true):Destroy() --Destroys each part
end

Anyways, I hope this helps. If not, or if you have any further questions, please leave a comment below, and I'll see what I can do.

I would suggest using this function I wrote, here is an example:

function SearchAndDestroy(obj,targ)
    for j,k in pairs(obj:GetChildren())do
        if k.Name == targ then
             k:Destroy()
        end
        SearchAndDestroy(k, targ)
    end
end

SearchAndDestroy(game.Workspace, "Decal")

Code:

function SearchAndDestroy(obj,targ)
    for j,k in pairs(obj:GetChildren())do
        if k.Name == targ then
             k:Destroy()
        end
        SearchAndDestroy(k, targ)
    end
end

Try this out. I commented some things, fixed the indentation (which is really important!), and added a way you could accomplish what you want. It's not the greatest method, but it should be okay for you to learn:

Debounce = false
block = script.Parent.Position

function touch(hit)
    if Debounce == false then
        Debounce = true
        wait(5)
        local bodyPositions = {} --lets keep track of them in a table shall we?
        local bh = Instance.new("Part", workspace)
        bh.Name = "bh"
        local function touched(bop)
            if bop.Anchored == false then
                local w = Instance.new("BodyPosition", bop)
                w.Name = "w"
                table.insert(bodyPositions, w) -- add this one to that table
                -- removed this statement because it's redundant (you just created w, no need to check): if bop:findFirstChild("w") then
                for i = 100, 1000, 10 do
                    w.D = i
                    w.P = i
                    w.Position = block
                    w.MaxForce = Vector3.new(i,i,i)
                    wait(.05)
                end
            end
        end
        bh.Touched:connect(touched) -- modified; you don't need to re-find bh since you already have the variable for it

        -- again no need to check. Removed: if workspace:findFirstChild("bh") then  
        bh.Shape = 0
        bh.BrickColor = BrickColor.new(1003)
        bh.CFrame = CFrame.new(block)
        bh.Anchored = true
        bh.CanCollide = false
        for i = 1, 50, 1 do
            bh.Size = Vector3.new(i,i,i)
            bh.CFrame = CFrame.new(block)
            wait(i/2000) -- this function will, at minimum, wait 1/30 of a second, or wait()
        end
        for i = 50, 1, -.5 do
            bh.Size = Vector3.new(i,i,i)
            bh.CFrame = CFrame.new(block)
            wait(i/6000) -- see above
        end
        -- remove everything in the table:
        for i, v in pairs(bodyPositions) do -- if you need explanation on this line, or more about tables, just ask
            if v then -- in case something got removed prematurely somehow
                v:Destroy()
            end
        end
        bh:Destroy()
        Debounce = false -- I also think this debounce used to be inside that 'if' statement, when it shouldn't have been
    end
end
script.Parent.Touched:connect(touch)